On the homogeneity region, growth modes and optoelectronic properties of chalcopyrite-type CuInS2

Studying the pseudo-binary phase diagrams of Cu2S-In2S3 and CuS-InS in the ternary system Cu-In-S showed that the CuInS2 compound semiconductor exhibits a melting point at T = 1093(+/- 3) degrees C and a homogeneity region at room temperature ranging from 2 mol% in In2S3 to 1.5 mol% in the CuS direction. This material, which crystallizes in the chalcopyrite-type structure, occurs in nature as the mineral roquesite. Investigating the influence of inert gas pressure on the crystallization of a stoichiometric melt, a disproportionation of the crystallized material into a Cu2S-rich bulk material of composition Cu1.06In0.98S2 epitaxially intergrown with similar to 1.5 mu m thick lamellae of the thiospinel phase CuIn5S8 was observed. Structure and composition of the bulk material were characterized by applying scanning Auger microscopy (SAM) and scanning electron microscopy (SEM) as well as differential thermal analysis (DTA) and X-ray diffractometry (XRD). In photoluminescence measurements (PL), Cu-rich bulk material is distinguished by sharp emission lines representing excitons close to the band to band transition at 1.55 eV and a broadband emission in the range from 1.3 eV to 1.45 eV. Exciton peaks of highest intensity were found at a Cu:In ratio= 1.0524. Investigating the spectral response of thin film chalcopyrite solar cells, a significant change of the optical gap from 1.45 eV for a "Cu-rich" to 1.37 eV for an "In-rich" absorber layer was observed. The results show that the homogeneity region of the material can be varied in the percentage range. A shift of the optical band gap can be influenced by stoichiometry (at% sulphur: at% metal) and the crystallization conditions of the adamantine-type structure.